Trophoblast cells from the placental villi represent the only type of human cell in which spontaneously budding type-C concornaviral particles have been seen with regularity by electron microscopy. This observation implies that the structural gene for complete type-C particle synthesis is present in at least some human cells, and that the perhaps generally prevailing pattern of suppression of expression by regulatory gene(s) is not absolute. The demonstration in primary C57BL/6 mouse brain cells that, following fusion to a permissive human line (Va2), expression of mouse type-C particles occurred in hybrid cells suggests that murine regulatory gene(s) were lost, presumably through cell fusion and resulting chromosome segregation. This conceptual and experimental background has led us to plan to apply somatic cell hybridization to the problem of oncornaviral induction in human placental cells. Over the past nine months we have traced the culture morphology, longevity of passage in culture, duration of expression of chronionic gonadotropin (HCG), and response to chemical oncornaviral inducing agents (e.g., iododeoxyuridine), of human placental cells in primary culture.